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Breaker sizing question

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Armourer

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#1
Well some of you may remember my previous post on converting a 220V mill to 110V, with my research on the mill it won't run on 110V. So I phoned my local electrician to come wire the proper 4 wire 220V plug for me, but was wondering how many amp breaker I should get them to install when they do it. The machine has a 1.5hp motor and the electrician said they would wire up a 30amp breaker. Seems a little over kill? What should I get them to install? Thanks all
 

kev74

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Well some of you may remember my previous post on converting a 220V mill to 110V, with my research on the mill it won't run on 110V. So I phoned my local electrician to come wire the proper 4 wire 220V plug for me, but was wondering how many amp breaker I should get them to install when they do it. The machine has a 1.5hp motor and the electrician said they would wire up a 30amp breaker. Seems a little over kill? What should I get them to install? Thanks all
To me, overkill is good. Especially if you decide to add more toys later.

The labor is the expensive part. A little bit heavier wire isn't a big expense for additional capacity.
 

Cadillac

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Here in the states for 220 your taking power from each leg of the 220 volt supply line. I think the smallest breakers are 10 amp so I believe your at 20amps minimum. If you want the the machine to be more protected closer to its motor rating you would have to put individual fuses or breakers in sub electrical boxes. What the electricians are wiring is a 220 line with a 30amp rating. Which is fine.
 

Ulma Doctor

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#4
go big (amps) as you wish to afford. you can always use a welder or larger piece of equipment
 

mksj

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The breaker at the panel protects the wire it is connected to, not necessarily the machine. A fuse or breaker at the machine, is then sized to protect the machine/wire. Although they can serve both purposes if the intent is to use the breaker for a machine on a dedicated circuit. The wire may be heavier than needed then the actual breaker used. So an example would be to use 10AWG wire, the breaker would be typically a 30A breaker, but a larger breaker is permitted if running a motor load which has a higher starting current, there are specific NEC guidance's as to what is permitted for motors and VFDs where the breaker is typically oversized by 125% of the motor FLC/FLA. There is often separate fusing and motor thermal overloads at the machine. So for my 5Hp compressor I was running #8 AWG to the compressor, and using a 50A panel breaker, the compressor had its own thermal overload relay at the machine contactor. Fuse and breakers are also rated with different curves and trip characteristics, so some handle short term high current loads better with out tripping/opening. The type varies by the load.
https://www.ecmweb.com/code-basics/motors-and-nec

If you are going through the effort to pull a new 230VAC circuit, then you should also consider what you may be running in future years, so I would recommend a minimum of 10AWG with a 30A panel breaker. The machine should have its own fusing, the panel breaker would be oversized relative to the motor load you intend to use. You may also consider running a heavier gauge wire and get an inexpensive 100A sub-panel and then pull individual dedicated circuits off of that as needed for your machinery. You also need to factor in length of wire and it's temperature rating, but this will all be covered by the electrician. Typically 230VAC machines are 3 wire, there is no neutral. A neutral would be used if you are pulling off a 120 VAC circuit at the machine, and this would need it's own local supplemental fusing/breaker. I often do this to run 120VAC drives, pumps and DRO. Many machines will use a 230:115VAC transformer instead to provide step down voltage, this also provides some degree of electrical/noise isolation, the transformer is separately fused for the maximum load.
 
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Bi11Hudson

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#6
First off, by the NEC the maximum load on a breaker shall never exceed 80%. That, of course is if a licensed electrician does the job. If the owner does it, without inspection, the NEC is more of a guideline. However, the NEC is a good guideline. If you follow it, you're sure of the safest install available. And, with electricity, that is not something to sneeze at. It really matters... ...

Now, 30 amp is about the minimum that I would use for myself. Think of it as the breaker protects the wiring, not the load. My compressor is set up on a 30 amp plug. It's only a 5 HP (compressor load). The motor has a thermo-disk in the end bell to protect the motor. The machine you're setting up should have an overload protection device integral to the controls. If not, there are many sources for such an animal.

Then, there is the 4 vs 3 wire business. If you are getting the 4 wire service, it probably is a sub-main. For that, I would want at least a 50 amp circuit. My shop has a 70 amp. With a seperate building and all the electrics in it, there is actually a 150 amp panel with a 70 amp feeder. Look at http://www.hudsontelcom.com/ , down the page a little for "Home Shop Electrics". It is lengthy, ~50 pages of PDF, but is well worth the time to read. For a non-electrical person, it should give you enough insight to keep from burning down the shop. An important point because the heat would damage the truth of the machine(s). And make the spouse a little angry, too.

Bill Hudson​
 
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